Filmmaking Tutorial: 180 Degree Rule and Other Shot Sequence Tips

This explains the 180 degree rule.

The 180-degree rule of shooting and editing keeps the camera on one side of the action. As a matter of convention, the camera stays on one side of the axis of action throughout a scene; this keeps characters grounded compositionally on a particular side of the screen or frame, and keeps them looking at one another when only one character is seen onscreen at a time. The technique allows for an expansion of the frame into the unseen space offscreen. It is referred to as a rule because the camera, when shooting two actors, must not cross over the axis of action; if it does, it risks giving the impression that the actors' positions in the scene have been reversed.